1. Field of the Invention
This invention relates to masks for lithography and more particularly to phase-shift masks for use in photolithography.
2. Description of related art.
In photolithography, masks are employed to expose a pattern upon a work piece. As manufacturing requirements call for exposure of patterns with smaller and smaller dimensions, it is becoming necessary to employ techniques which permit enhancement of the current performance of the process of photolithography. One trend has been to use electromagnetic energy with shorter wavelengths in the UV wavelengths, x-rays and the like. An alternative approach is to use phase-shifting techniques in the ranges of wavelengths used in photolithography in the past.
The phase-shifting technique can improve the resolution of coherent or partially coherent optical imaging systems. It has been shown that the normalized resolution k1 can be reduced from 0.7 to 0.35 to improve lithography by two generations.
Use of phase-shifting techniques with masks in optical microlithography systems has been shown to improve projected images. These phase-shifting techniques have been practiced in various configurations as described in the following references.
Nitayama et al, "New Phase Shifting Mask with Self-Aligned Phase Shifters for Quarter Micron Photolithography", IEDM Technical Digest, pp. 57-60 (1989) shows a phase shifting mask (PSM) pattern consisting of a phase-shifting layer and a slightly smaller opaque layer which blocks light at the center of the mask pattern. This mask uses the rim (RIM), i.e. border of the structure, to form a phase shift in the image passing through the mask. In the RIM phase-shifting mask of Nitayama et al, a given mask pattern consists of a substrate carrying a phase-shifting layer and a slightly smaller opaque layer 11 which blocks light at the center of the mask pattern. This arrangement lends itself to strong enhancement of edge contrast due to the phase-shifting, yet prevents a large negative amplitude at the center of patterns to form ghost lines.
The prior art publication by Nitayama et al, above, teaches delineation of the phase-shifting layer on top followed with a self-aligned overetch of the blocking layer. The undercutting dimension and profile are difficult to control for the precision required to maintain linewidth control of the blocking pattern.
Nakagawa et al "Fabrication of 64M DRAM with i-Line Phase-shift Lithography", IEDM 90, pages 817-820 (1990) shows in FIGS. 2(a) and (b) the mask of blocker layer above a phase shift layer, with a chromium blocker having narrower dimensions than the Phase shift layer.
Levenson et al, "Improving Resolution in Photolithography with a Phase-Shifting Mask", IEEE Transactions on Electron Devices, Vol. ED-29, No. 12, pp1828-1836, (Dec. 1982) describes alternating element phase-shifting.
U. S. Pat. No. 5,045,417 of Okamoto for "Mask for Manufacturing Semiconductor Device and Method of Manufacture Thereof" shows (in FIG. 12 thereof) a synthetic quartz glass substrate (2) covered with a metal layer (3) (described as Embodiment 4 in Col. 13, lines 28-62.) A phase-shifting groove (7b) of depth (d) is shown in the quartz glass adjacent to openings in the metal layer (3.)
Provision of phase shift repair layers is described in commonly assigned, copending application Ser. No. 07/886,651, filed Apr. 8, 1992 (FI9-91-119) by Burn Jeng Lin for "A Mask Structure and Process to Repair Missing or Unwanted Phase Shifting Elements." Furthermore, contacting the phase shifters to the substrate 10 reduces multiple reflections at the interfaces.
Descriptions of three methods of fabricating a shifter-based (SB) RIM phase-shifting (PS) mask are described below in commonly assigned, copending U.S. patent application, Ser. No. 07/872,781 (FI9-91-143) of Lin for "A Shifter-Based Rim Phase Shifting Structure and Process to Fabricate the Same."
The alternating-element phase-shifting mask (PSM) by M. D. Levenson, N. S. Viswanathan, and R. A. Simpson, IEEE Trans. Electron Devices, Vol. ED-29, p. 1828, (1982) has the potential to double the resolution of a given optical imaging system. It is the most effective PSM for closely packed structures. Its isofocus characteristics are also very desirable; B. J. Lin, SPIE Proceedings Vol. 1496, p. 54, (1990). However, when there is variable packing in the mask, the alternating-element PSM is less effective. In addition, it does not work on isolated openings or opaque features.
The rim PSM is described in "Fabrication of 64DRAM with i-line phase-shift lithography" by K. Nakagawa, M. Taguchi and T. Ema, IEDM 90, pages 817 to 820 (1990). Rim PSM works on arbitrary mask patterns but its improvement for closely packed patterns is minimal.
In accordance with this invention, an ALRIM PSM consists of rim phase shifters and reversed rim phase shifters. Preferably, absorptive blockers are present and the absorptive blockers are removed for originally narrow isolated absorber lines. The reversed rim phase shifters can be placed alternately with the normal rim phase shifters. The normal rim phase shifters and reverse rim phase shifters can be exchanged. The reversed rim phase shifters can be placed alternately with the normal rim phase shifters. The device can be built on a substrate consisting of an absorber layer on a distinct phase shifter layer on the mask substrate, on a substrate consisting of an absorber layer on the mask substrate, or built on a substrate consisting of an absorber layer on a distinct phase shifter layer on the mask substrate. The device can be built on a substrate consisting of an absorber layer on the mask substrate, or on a substrate consisting of an absorber layer on a distinct phase shifter layer on the mask substrate. Preferably 360.degree. phase shift is used in combination of 0.degree. and 180.degree. built on a substrate consisting of an absorber layer on two distinct phase shifter layers on the mask substrate. Preferably, one of the distinct phase shifter layers 18 is of similar material to that of the mask substrate. The device can be built on a substrate consisting of an absorber etch mask layer on an absorber layer on the mask substrate, or built on a substrate consisting of an absorber etch mask layer on an absorber layer on the mask substrate.
The ALRIM PSM can have the patterns adjusted by a constant size change. The size change can be adjusted individually and the size change can be intra-feature.
In another aspect of the invention, an ALRIM PSM fabrication process comprises
a. a two-level resist exposure, PA1 b. selective removal of absorber masked by resist after first development, PA1 c. selective removal of phase shifter masked by absorber, PA1 d. further selective removal of absorber masked by resist after second development PA1 a. formation of a T-shaped etch mask structure PA1 b. delineation of all normal rim phase shifter by the T-shaped structure, and PA1 c. delineation of reversed rim phase shifters by second and third aligned resist images. PA1 a. a standard process to fabricate all features into normal rim phase shifter. PA1 b. a second aligned exposure on a new resist layer to define the reversed rim areas for etching. PA1 a. a two-level exposure process, PA1 b. selective etching of an absorber etch mask layer masked by the first developed resist image PA1 c. second resist development, PA1 d. selective etching of the absorber masked by the absorber etch mask PA1 e. selective etching of the absorber etch mask masked by the second resist image. PA1 f. second selective etching of the absorber masked by the absorber etch mask. PA1 a. T-shaped etch mask structure PA1 b. selective etching of an absorber etch mask layer masked by the top of the T-structure PA1 c. Removal of the top of the T-structure PA1 d. selective etching of the absorber masked by the absorber etch mask, PA1 e. Selective etching of the absorber etch mask masked by bottom of the T Structure PA1 f. Second selective etching of the absorber masked by the absorber etch mask
Alternatively, an ALRIM PSM fabrication process comprises:
In another alternative, an ALRIM PSM fabrication process comprises:
The ALRIM PSM fabrication process can also comprise:
An ALRIM PSM fabrication process in accordance with this invention comprises formation of:
A wafer image formation process using an ALRIM PSM mask consists of rim phase shifters and reversed rim phase shifters. Preferably the mask includes absorptive blockers and the absorptive blockers are removed for originally narrow isolated absorber lines. Preferably, the reversed rim phase shifters are placed alternately with the normal rim phase shifters; the normal rim phase shifters and reverse rim phase shifters are exchanged.
Alternatively, the reversed rim phase shifters are placed alternately with the normal rim phase shifters, and the normal rim phase shifters and reverse rim phase shifters are exchanged.